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Regional coordinate systems do usually not follow tectonic movements. Differences between the ITRF2008 and the regional coordinate systems might be up to several decimeters (for example, in Europe). Therefore, it is necessary for Trimble 4D Control Server to consider the necessary transformation equations between the coordinate systems.
The International Earth Rotation Service (IERS) maintains the global International Terrestrial Reference System (ITRS) and the International Terrestrial Reference Frame (ITRF). The ITRS is the theoretical definition of a coordinate system with respect to the fixed stars. The Reference Frame is a realization (computation) of this Reference System using a specific set of observations and a selection of observing stations. It is in practice realized by the three-dimensional determination and adjustment of the coordinates and motions of reference stations in a global network using satellite geodesy methods, such as Satellite Laser Ranging, Lunar Laser Ranging, Very Long Baseline Interferometry and GPS.
Various coordinate/velocity sets (reference frames) exist representing different realizations of ITRS. These sets are named according to the year of their realization, for example ITRF2000, ITRF2008. For more detailed information, refer to the following websites:
Intercontinental, continental and local reference systems are derived from the ITRS; for example, the ETRS89 was realized by defining the European Reference Frame 89 (ETRF89), which is identical to ITRF89 at epoch 1989.0. Trimble 4D Control Server comes with a selection of global and intercontinental reference frames.
To transform the coordinates from one reference frame to another, a 7-parameter similarity transformation model is applied in combination with velocities modeling relative motions (translation rates, rotation rates, and scale rate). To account for crustal motion of tectonic plates, plate rotation rates are available for all reference frames.